1. Field of the Invention
The invention relates to an MR METHOD WITH FIELD GRADIENTS SELECTED ACCORDING TO THE DISPLACEMENTS OF AN OBJECT in which the displacement of a pulsating object, or a part thereof, present in an examination zone is continuously measured with respect to a reference position and in which the reconstruction of an MR image utilizes exclusively MR signals acquired from the examination zone while the displacement from the reference position reaches or falls below a threshold value. The invention also relates to an arrangement for carrying out such a method.
2. Description of the Related Art
A method and arrangement of this kind are known from a publication by Sachs et al. in the magazine MRM 33, pp. 639-645 (1994). In the case of prolonged MR (magnetic resonance) examinations, motion artefacts occur notably due to respiratory movements of the patient to be examined. Parts of the body of the patient then perform a reciprocating (pulsating) movement which causes motion artefacts in an MR image reconstructed by means of the MR signals acquired during such examinations.
In the known method the respiratory movement is continuously measured by means of so-called navigator pulses. The reconstruction of an MR image utilizes the MR signals generated in the examination zone while the respiration-induced displacement of the object to be examined (or a part thereof) from a reference position does not exceed a threshold value. According to the known method the examination is performed by means of so-called spiral-MRI, but it can also be carried out in conjunction with other MR sequences.
FIG. 1 shows how MR signals are acquired with different phase encoding according to the known method. Therein, it is assumed that the phase encoding is produced by a magnetic gradient field having a gradient acting in the y-direction and that the MR signals, or the nuclear magnetization in the examination zone, are acquired with a given temporal sequence of the phase encoding or of the time integral over the phase coding gradient (this integral is customarily denoted by the reference k.sub.y). Thus, the ordinate represents the relevant k.sub.y value whereas the abscissa represents the time or the number of excitations of the nuclear magnetization in the examination zone, it being assumed that these excitations are repeated after a given period of time (for example, 15 ms). Moreover, below the coordinate there is shown the displacement of the object, or a part thereof, as a function of time. Even though MR signals are generated by continuous excitation according to the known method, not all signals are stored or used for the reconstruction of an MR image. Only when the displacement v drops below a predeterminable threshold value the MR signals generated in the examination zone are acquired and stored while the absolute value of the phase encoding increases. This is represented by small squares in FIG. 1; each square represents a given phase encoding or an MR signal acquired and stored with this phase encoding.
When the displacement exceeds the threshold value, the MR signals occurring are no longer acquired because their processing would cause motion artefacts in the MR image. It is only after a further respiratory period that a state is reached again in which the MR signals, with a different phase coding, can be acquired and stored again. This is repeated for each respiratory period until an MR signal has been acquired and stored for each of the, for example, 128 different k.sub.y values.
Assuming that the body of the patient is for only 25% of each respiratory period in a movement phase w which does not, or not significantly, cause motion artefacts in the MR image, the 128 MR signals required for the reconstruction of an MR image with reduced motion artefacts will have been acquired and stored only after more than 500 excitations of the nuclear magnetization zone. Therefore, in order to reduce the examination time it is known (Wood et al. in the magazine Med. Phys. 13 (6), pp. 794 ff. (1986)) to limit this "gating" either to the high spatial frequencies or to the low spatial frequencies, the best results being offered by the limitation to the gating of the high spatial frequencies.
The "gating" of the MR signals thus prolongs the period of time required for the acquisition of all MR signals. The smaller the displacement from the reference position that is tolerated for the acquisition of the MR signals, the more the movement artefacts will be reduced, but also the longer the examination time will be. Therefore, in this method a compromise must be made between image quality and examination time.